In the manufacture of composite cans, several inner layers of material and an outer printed label are spirally wound around a mandrel in order to form a composite tube. The tube moves down the mandrel and is cut into lengths, called sticks, by means of a pair of knives carried on a reciprocating carriage which moves in synchronization with the tube. The motion of the carriage is synchronized with that of the tube by tracking the motion of the printed label before it is wound on the mandrel and controlling the motion of the carriage as a function of the label travel. The label motion is tracked by detecting the passage of a plurality of evenly spaced reference marks, called eyemarks, past on inspection point. The eyemarks are located on a track which contrasts with the color of the eyemarks. A scanner is used to detect a passage of the eyemarks by responding to a change in the amount of light which is reflected from the eyemarks with respect to the amount of light which is reflected from the contrasting track. Such scanners are well known in the art and include a light source which projects light towards the track and a photodetector which receives light which is reflected from the track. The output of the scanner is a function of the amount of light received by the photodetector.
In prior art scanning systems, the output of the photodetector is fed to a circuit which generates a pulse whenever an eyemark is detected. The prior art scanners however, are generally designed to work only for a particular color of eyemarks and track. If different colors are to be utilized, the scanner must be adjusted or switched whenever a new color is to be used, due to different reflectivities of different colors. Composite cans are presently manufactured with many different color combinations for the track and eyemarks. Often, these combinations will be run on the same machine, i.e., a job change will require a new label to be run on a machine which was previously running with a different type of label. With such a change, the prior art scanners have to be adjusted or switched so as to accomodate the different color combinations. For example, many labels are printed with white tracks and black eyemarks. If it is desired to run a label having a blue track and white eyemarks, the scanner must be adjusted to compensate for the different reflectivities between the two labels.
The present invention is a scanner which includes a circuit that enables the scanner to be utilized with a wide range of color combinations. It adjusts itself automatically depending upon the average reflectivity of the surface and depending upon the amount of contrast between the track and eyemarks. Whenever too much light is being reflected back to the photodetector, the intensity of the scanner light source is automatically reduced, so as to prevent saturation of the photodetector. In order to detect the passage of an eyemark the circuit determines when a change in the amount of light received by the photodetector exceeds a predetermined value. This transistion represents the change between the track and an eyemark. If the contrast between the track and the eyemark is very large, the change in the output of the photodetector will be correspondingly large and will exceed a second predetermined value and automatically cause the circuit to reduce its sensitivity. Thus, the circuit automatically adjusts itself depending upon the reflectivity of the surface and depending upon the contrast between the track and the eyemarks. The circuit also includes means for resetting itself to maximum sensitivity whenever a a predetermined time period passes without the sensing of an eyemark.